Low Fuel Pressure on a 98 Ford Explorer

[Thomas E. Shepherd]

Hello All,

I have a 98 Ford Explorer that has Low Fuel Pressure, both indicated
by the OBD codes and my direct measure with a fuel pressure guage ~25
psi while spec is ~35 psi. I have followed the diagnostic procedure
outlined in a Haynes maintenance manuel ( minus "back-probing" one of
the electrical connectors which I could not figure out how to do ).

The fuel pump seems to be good as when I pinch the return line, fuel
pressure goes up to ~65 psi.

The fuel pressure regulator holds pressure at ~35 psi when the engine
is turned off.

Fuel pressure rises from ~25 psi to ~35 psi ( spec ) when I disconnect
the vacuum line from the fuel pressure regulator.

Vacuum at the fuel pressure regulator is -15 mmHg (?) I can't remember
the units.

The engine was replaced a couple of years ago ( under warantee :) due
to the plastic timing chain tensioner vaporizing, and the more I work
on the engine, the more I notice missing bracket bolts etc.

I could replace the fuel pressure regulator again, but I do not
suspect that this is truely the problem. I have a hunch that it may
be a incorrect vacuum at the fuel pressure regulator. If I understand
correctly, the vacuum is supposed to change with engine speed and
modify how much gas is available, i.e. when the engine is idling, less
gas is required so the fuel pressure regulator is wide open to allow
the gas to flow back to the gas-tank. And when the engine is at high
RPMs, more gas is needed so the regulator closes down and restricts
the amount of gas returning to the gas-tank.

Help, any suggestions or ideas will be appreciated. I am at a loss
what to do next.

Thanks ~ Thomas E. Shepherd

[Bruce Chang]

sounds like a dirty fuel filter.

I don't see where you're getting low fuel pressure if you say that it runs
at ~35 with the vac line disconnected and spec is ~35..?

The fuel pressure regulator doesn't give the engine any more gas, it only
stabilizes the pressure across the injector. As intake vacuum decreases,
the pressure on the fuel side needs to increase to meter the same amount of
fuel. As intake vaccum increases, it will assist the fuel injector in
sucking the fuel into the intake, rather than fuel pressure being the only
force behind the gas.

-Bruce

"Thomas E. Shepherd" <thomas.e...@boeing.com> wrote in message
news:1f7c957b.02121...@posting.google.com...

[Thomas E. Shepherd]

Hello All,

I presented the situation poorly. Fuel pressure at idle, with all
vacuum lines properly connected, is about 25psi where the specified
fuel pressure should be greater than 35psi. So I'm about 10psi shy if
spec.

When I disconnect the vacuum line to the regulator, the fuel pressure
then rises to about 35psi when the specified fuel pressure should be
around 45psi. I don't have the manuel with me, so I might be off on
the numbers, but all measurements indicated low fuel pressure.

Thanks for the clarification about the fuel pressure regulator. I've
been reading quite abit on these newsgroups and have learned what it
is that the fuel pressure regulator does, and now understand that it
is maintaining a constant pressure differential accross the fuel
injectors.

I changed the fuel filter at the time I made the pressure measurements
...

Any other ideas ... ?

Thanks, Tom Shepherd

"Bruce Chang" <bec...@swbellnospam.com> wrote in message news:<DQJL9.199$6B3.16...@newssvr12.news.prodigy.com>...

[David J and Lynne J Shepherd]

You may want to make sure, based on the vacuum hose routing diagram under
the hood,that the regulator is connected to the proper source, if so and
your guage is accurate, seems like the reguator calibration is off, which of
course I don't think is adjustable.

"Thomas E. Shepherd" <thomas.e...@boeing.com> wrote in message
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[Bruce Chang]

If it's not the fuel pressure regulator (FPR) then I'd suggest it's your
fuel pump.. You should do a volume test and see if the fuel pump is
supplying the correct volume of fuel. I suspect it will not with a low fuel
pressure like that I'm looking online and one site says 1 pint for 30
seconds.. Someone chime in please if you know for sure.

I'd also see how quickly your fuel pressure drops after the system is
pressurized.. I think with my firebird, the fuel pressure does not drop
significantly while watching it. Applying power to an injector drops the
pressure quickly. I'd say 2-3 psi with just a tap.

Low fuel pressure can only be effected by the fuel pump, fuel filter, fuel
regulator, injectors and the lines themselves.. It's also possible that the
fuel pump screen has become clogged. I didn't think about that until now..
I think the volume test should be your next action.

-Bruce

"Thomas E. Shepherd" <thomas.e...@boeing.com> wrote in message

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